The question of existence in "As I lay dying"

"In a strange room you must empty yourself for sleep. And before you are emptied for sleep, what are you. And when you are emptied for sleep, what are you. And when you are emptied for sleep you are not. And when you are filled with sleep, you never were. I don't know what I am. I don't know if I am or not. Jewel knows he is, because he does not know that he does not know where he is or not. He cannot empty himself for sleep because he is not what he is and he is what he is not. Beyond the unlamped wall I can hear the rain shaping the wagon that is ours, the load that is no longer theirs that felled and sawed it nor yet theirs that bought it and which is not ours either, lie on our wagon though it does, since only the wind and the rain shape it only to Jewel and me, that are not asleep. And since sleep is is-not and rain and wind are was, it is not. Yet the wagon is, because when the wagon is was, Addie Bundren will not be. And Jewel is, so Addie Bundren must be. And then I must be, or I could not empty myself for sleep in a strange room. And so if I am not emptied yet, I am is.
How often have I lain beneath rain on a strange roof, thinking of home." (Darl) 

"'Jewel's mother is a horse', Darl said. 
'Then mine can be a fish, cant it, Darl?' I said. 
Jewel is my brother. 
'Then mine will have to be a horse, too', I said.
'Why?' Darl said. 'If pa is your pa, why does your ma have to be a horse just because Jewel's is?' 
'Why does it?' I said. 'Why does it, Darl?' 
Darl is my brother. 
'Then what is your ma, Darl?' I said. 
'I haven't got ere one', Darl said. 'Because if I had one, it is was. And if it is was, it cant be is. Can it?' 
'No', I said. 
'Then I am not', Darl said. 'Am I?' 
'No', I said. 
I am. Darl is my brother. 
'But you are, Darl', I said. 
'I know it', Darl said. 'That's why I am not is. Are is too many for 1 woman to foal.'" (Vardaman) 

Immersed in grief and despair and doubt and disappointment with his parents' hypocrisy and deceit, Darl grows irrational and questions his own existence and loses his sense and becomes alienated from his own self. 
And finally writes in 3rd-person. 
As though he's someone else.